Tomcat ClassLoader研究

最新推荐文章于 2021-12-14 13:07:39 发布

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最新推荐文章于 2021-12-14 13:07:39 发布

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分类专栏： Java相关文章标签： classloader tomcat class file exception null

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本文详细解析了Tomcat的ClassLoader层次结构及工作原理，包括Bootstrap、System、Common、CatalinaShared和Webapp等ClassLoader的创建过程。同时介绍了WebappClassLoader的findClass方法如何改变传统搜索顺序。

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http://tomcat.apache.org/tomcat-4.1-doc/class-loader-howto.html

Tomcat的ClassLoader层次结构：


	Bootstrap \| System \| Common / / Catalina Shared / / Webapp1 Webapp2 ...

源代码如下：

org.apache.catalina.startup.Bootstrap类（tomcat主类）

public final class Bootstrap { // ------------------------------------------------------- Static Variables /** * Debugging detail level for processing the startup. */ private static int debug = 0; // ----------------------------------------------------------- Main Program /** * The main program for the bootstrap. * * @param args Command line arguments to be processed */ public static void main(String args[]) { // Set the debug flag appropriately for (int i = 0; i < args.length; i++) { if ("-debug".equals(args[i])) debug = 1; } // Configure catalina.base from catalina.home if not yet set if (System.getProperty("catalina.base") == null) System.setProperty("catalina.base", getCatalinaHome()); // Construct the class loaders we will need ClassLoader commonLoader = null; ClassLoader catalinaLoader = null; ClassLoader sharedLoader = null; try { File unpacked[] = new File[1]; File packed[] = new File[1]; File packed2[] = new File[2]; ClassLoaderFactory.setDebug(debug); unpacked[0] = new File(getCatalinaHome(), "common" + File.separator + "classes"); packed2[0] = new File(getCatalinaHome(), "common" + File.separator + "endorsed"); packed2[1] = new File(getCatalinaHome(), "common" + File.separator + "lib"); commonLoader = ClassLoaderFactory.createClassLoader(unpacked, packed2, null); unpacked[0] = new File(getCatalinaHome(), "server" + File.separator + "classes"); packed[0] = new File(getCatalinaHome(), "server" + File.separator + "lib"); catalinaLoader = ClassLoaderFactory.createClassLoader(unpacked, packed, commonLoader); unpacked[0] = new File(getCatalinaBase(), "shared" + File.separator + "classes"); packed[0] = new File(getCatalinaBase(), "shared" + File.separator + "lib"); sharedLoader = ClassLoaderFactory.createClassLoader(unpacked, packed, commonLoader); } catch (Throwable t) { log("Class loader creation threw exception", t); System.exit(1); } Thread.currentThread().setContextClassLoader(catalinaLoader); // Load our startup class and call its process() method try { SecurityClassLoad.securityClassLoad(catalinaLoader); // Instantiate a startup class instance if (debug >= 1) log("Loading startup class"); Class startupClass = catalinaLoader.loadClass ("org.apache.catalina.startup.Catalina"); Object startupInstance = startupClass.newInstance(); // Set the shared extensions class loader if (debug >= 1) log("Setting startup class properties"); String methodName = "setParentClassLoader"; Class paramTypes[] = new Class[1]; paramTypes[0] = Class.forName("java.lang.ClassLoader"); Object paramValues[] = new Object[1]; paramValues[0] = sharedLoader; Method method = startupInstance.getClass().getMethod(methodName, paramTypes); method.invoke(startupInstance, paramValues); // Call the process() method if (debug >= 1) log("Calling startup class process() method"); methodName = "process"; paramTypes = new Class[1]; paramTypes[0] = args.getClass(); paramValues = new Object[1]; paramValues[0] = args; method = startupInstance.getClass().getMethod(methodName, paramTypes); method.invoke(startupInstance, paramValues); } catch (Exception e) { System.out.println("Exception during startup processing"); e.printStackTrace(System.out); System.exit(2); } }

其中：

commonLoader =
ClassLoaderFactory.createClassLoader(unpacked, packed2, null);

创建common classloader，以AppClassLoader为父ClassLoader

catalinaLoader =
ClassLoaderFactory.createClassLoader(unpacked, packed,
commonLoader);

创建catalina classloader，以common classloader为父classloader

sharedLoader =
ClassLoaderFactory.createClassLoader(unpacked, packed,
commonLoader);

创建share classloader，以common classloader为父classloader

Thread.currentThread().setContextClassLoader(catalinaLoader);

设置ContextClassLoader为catalina classloader

org.apache.catalina.startup.ClassLoaderFactory类

public static ClassLoader createClassLoader(File unpacked[], File packed[], ClassLoader parent) throws Exception { if (debug >= 1) log("Creating new class loader"); // Construct the "class path" for this class loader ArrayList list = new ArrayList(); // Add unpacked directories if (unpacked != null) { for (int i = 0; i < unpacked.length; i++) { File file = unpacked[i]; if (!file.isDirectory() || !file.exists() || !file.canRead()) continue; if (debug >= 1) log(" Including directory " + file.getAbsolutePath()); URL url = new URL("file", null, file.getCanonicalPath() + File.separator); list.add(url.toString()); } } // Add packed directory JAR files if (packed != null) { for (int i = 0; i < packed.length; i++) { File directory = packed[i]; if (!directory.isDirectory() || !directory.exists() || !directory.canRead()) continue; String filenames[] = directory.list(); for (int j = 0; j < filenames.length; j++) { String filename = filenames[j].toLowerCase(); if (!filename.endsWith(".jar")) continue; File file = new File(directory, filenames[j]); if (debug >= 1) log(" Including jar file " + file.getAbsolutePath()); URL url = new URL("file", null, file.getCanonicalPath()); list.add(url.toString()); } } } // Construct the class loader itself String array[] = (String[]) list.toArray(new String[list.size()]); StandardClassLoader classLoader = null; if (parent == null) classLoader = new StandardClassLoader(array); else classLoader = new StandardClassLoader(array, parent); classLoader.setDelegate(true); return (classLoader); }

ClassLoaderFactory创建的是StandardClassLoader(org.apache.catalina.loader包中)

由Bootstrap类调用Catalina类的process()方法，再调用execute()方法，再调用start()来启动Server实例。

当Tomcat开启每个Context时，是调用的StandardContext的start()方法，其中：

设置Loader为WebappLoader

if (getLoader() == null) { // (2) Required by Manager if (getPrivileged()) { if (debug >= 1) log("Configuring privileged default Loader"); setLoader(new WebappLoader(this.getClass().getClassLoader())); } else { if (debug >= 1) log("Configuring non-privileged default Loader"); setLoader(new WebappLoader(getParentClassLoader())); } }

然后调用：bindThread()，设置当前线程的ClassLoader为WebappLoader的ClassLoader，即为WebappClassLoader

private ClassLoader bindThread() { ClassLoader oldContextClassLoader = Thread.currentThread().getContextClassLoader(); if (getResources() == null) return oldContextClassLoader; Thread.currentThread().setContextClassLoader (getLoader().getClassLoader()); DirContextURLStreamHandler.bind(getResources()); if (isUseNaming()) { try { ContextBindings.bindThread(this, this); } catch (NamingException e) { // Silent catch, as this is a normal case during the early // startup stages } } return oldContextClassLoader; }

WebappLoader的ClassLoader的赋值如下：

private String loaderClass = "org.apache.catalina.loader.WebappClassLoader"; ...... public void start() throws LifecycleException { // Validate and update our current component state if (started) throw new LifecycleException (sm.getString("webappLoader.alreadyStarted")); if (debug >= 1) log(sm.getString("webappLoader.starting")); lifecycle.fireLifecycleEvent(START_EVENT, null); started = true; if (container.getResources() == null) return; // Register a stream handler factory for the JNDI protocol URLStreamHandlerFactory streamHandlerFactory = new DirContextURLStreamHandlerFactory(); try { URL.setURLStreamHandlerFactory(streamHandlerFactory); } catch (Throwable t) { // Ignore the error here. } // Construct a class loader based on our current repositories list try { classLoader = createClassLoader(); classLoader.setResources(container.getResources()); classLoader.setDebug(this.debug); classLoader.setDelegate(this.delegate); if (container instanceof StandardContext) classLoader.setAntiJARLocking(((StandardContext) container).getAntiJARLocking()); for (int i = 0; i < repositories.length; i++) { classLoader.addRepository(repositories[i]); } // Configure our repositories setRepositories(); setClassPath(); setPermissions(); if (classLoader instanceof Lifecycle) ((Lifecycle) classLoader).start(); // Binding the Webapp class loader to the directory context DirContextURLStreamHandler.bind ((ClassLoader) classLoader, this.container.getResources()); } catch (Throwable t) { throw new LifecycleException("start: ", t); } // Validate that all required packages are actually available validatePackages(); // Start our background thread if we are reloadable if (reloadable) { log(sm.getString("webappLoader.reloading")); try { threadStart(); } catch (IllegalStateException e) { throw new LifecycleException(e); } } } ...... private WebappClassLoader createClassLoader() throws Exception { Class clazz = Class.forName(loaderClass); WebappClassLoader classLoader = null; if (parentClassLoader == null) { // Will cause a ClassCast is the class does not extend WCL, but // this is on purpose (the exception will be caught and rethrown) classLoader = (WebappClassLoader) clazz.newInstance(); } else { Class[] argTypes = { ClassLoader.class }; Object[] args = { parentClassLoader }; Constructor constr = clazz.getConstructor(argTypes); classLoader = (WebappClassLoader) constr.newInstance(args); } return classLoader; }

在WebappClassLoader中，其findClass的搜索顺序与一般的ClassLoader的搜索顺序不同。

一般的ClassLoader的搜索顺序为：

将其委托给父ClassLoader，如果父ClassLoader不能载入相应类，则才交给自己处理

但是WebappClassLoader中，其是先由自己来处理，如果不行再委托给父ClassLoader